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Structure of poly-bis-trifluoroethoxyphosphazene fibres
Polymer Science U.S.S.R. Vol. 32, No. 1, pp. 108-115, 1990 Printed in Great Britain.
0032-3950/90 $10.00 + .00 © 1991 Pergamon Press plc
STRUCTURE OF POLY-bisTRIFLUOROETHOXYPHOSPHAZENE FIBRES* E. M. ANTIPOV, V. G. KULICHIKHIN, L. K. GOLOVA, N. P. KRUCHININ, D. R. TUR and N. A. PLATt~ "Khimvolokno" Scientific Production Association; and A. N. Nesmeyanov Institute of Heteroorganic Compounds, Academy of Sciences of the U.S.S.R.
(Received 1 August 1988)
The structural transformations during phase transitions of poly-b/s-trifluoroethoxyphosphazene fibres during heating-cooling cycles are studied by X-ray diffraction. The evolution of the changes in structural parameters is followed as a function of the stretching and heat treatment conditions. It is shown that the defective character of the system is largely determined by the fibre annealing temperature. It is assumed that the polymer macromolecules are spontaneously straightened in the mesomorphic state.
THE USE of polymers as fibres provides a means of solving a great variety of problems in establishing specialized composite materials. Poly-b/s-trifluoroethoxyphosphazene (PP) is a multi-purpose polymer, often used in the form of fibres and fabrics. Since the properties of fibrous materials are largely determined by the special features of their structural organization, the necessity of studying the structure of PP as a function of the degree of orientation attained in cold and hot stretching is obvious. In view of this, the main problem in this work is a study of the special structural features of fibres obtained from a solution of high molecular PP as a function of the stretching and heat treatment conditions. The method described by Vinogradova et al. [1] was used to synthesize the PP. The molecular weight, as evaluated from the intrinsic viscosity of a solution in THF[2] was 1.2x 107; the polydispersity factor Mw/Mn, as determined from the MMD distribution curve did not exceed 1.4. The PP fibres were obtained by a wet method from a 3% solution in ethyl acetate. A mixture of CC14 and ethyl acetate was used as precipitant. The dried PP fibres were subjected to annealing and orientational stretching at various temperatures. The conditions of obtaining the specimens are shown in Table 1. In the X-ray diffraction studies, the large angle photo-X-ray diffraction patterns were obtained with a IRIS-3.0 apparatus (CuK~, Ni filter, fiat cell). The diffraction patterns were obtained with a DRON-3.0 instrument (CuK~, curved, focusing quartz crystal-monochromator in the primary beam) using transillumination photography. The diffraction patterns were recorded along the equator and along the meridian of the photo-X-ray patterns. The photographs of the oriented specimens at different temperatures were obtained under isometric conditions; the fibres were wound onto a special frame-holder and fixed with clamps at the edges. The small angle diffraction patterns were obtained with a KRM-1 apparatus with an automated system for controlling and processing the data. Photography was carried out with slit collimation along the meridian of the X-ray patterns, at a resolution of five angular minutes. * Vysokomol. soyed. A32: No. 1, 108-115, 1990.
108
Structure of PP fibres
109
Depending on the molecular mass and the conditions of obtaining the material the PP can crystallize with the formation of three types of crystal lattice, i.e. a,/3, and y [3, 4]. It is shown that the a- and y-phases are orthorhombic in type with two and four monomer units in the elementary cell, respectively, whereas the /3-modification is monoclinic, which is characteristic only of the crystals of low molecular PP. In the region of 343-520 K the PP is in the mesophase state.
TABLE 1.
Specimen number 1 2 3 4 5 6 7 8
CHARACTERISTICS OF PP SPECIMENS
Specimen characteristics
Isotropicfilm from solution Fibre without orientationalstretching Specimen2, subjected to orientationai stretchingat 293 K Specimen3 after annealingat 433 K on a rigid frame over 1 hour Specimen3, subjected to hot stretching at 448 K Specimen3, subjected to hot stretching at 453 K Specimen6, subjectedto hot stretchingat 473--483 K Specimen7, subjected to hot stretching at 523 K
Extension ratio, ;t
Azimuthal half-width A~, deg
0 0 3.4 3.4 6.5 6.8 8.0 10-12
-30 7.5 17.5 9.0 8,5 8.0 7.5
The wide angle diffraction patterns of specimen 1 (Table 1) indicate the partially crystalline nature of the polymer film obtained by casting from solution. Analysis of the diffraction data showed that in this case PP is crystallized in a non-equilibrium a-lattice. The process evidently involves by-passing the preliminary stage in which a mesophase appears, in contrast to crystallization of the polymer from the melt with a formation of a y-phase. In the first case relatively small crystallites are obtained with a-orthorhombic packing of the chains, which, according to previous authors [3, 4] are in the folded conformation. The temperature of transition of the a-phase into the mesomorphic state is - 2 0 K lower than the corresponding temperature of the equilibrium y-modification. There is no preferential orientation of the macromolecules in the PP films. Evaluation of the dimensions of the crystalline formations from the half-widths of the diffraction lines gives the value 15-20 nm. According to data provided by Magill et al. [15] this polymer has spherulitic supermolecular organization. Determination of the degree of crystallinity is difficult because of the impossibility of separating the scattering from the amorphous and crystalline components. The half-width of the profile of the amorphous halo is probably relatively small, and is screened by the crystal reflections. The density of crystalline packing of the a-phase, as determined from crystallographic data at 293 K, is 1780 kg/m 3, i.e. somewhat higher than for the y-modification (1732 kg/m3), which is natural for a system with half the elementary cell. However, because of the low degree of crystalinity of the non-equilibrium a-lattice the overall density of specimens in the various cases is approximately the same, and according to Refs [3, 4] is 1715 kg/m 3. A PP fibre formed from solution without plastificational stretching (specimen 2) shows weak signs of anisotropy, which indicates the initial stage of molecular orientation. Uniaxial stretching of such a specimen at 293 K results in the appearance of an axial texture (specimen 3) (Fig. la, and Fig. 2, diffraction pattern 1). It should be noted that after stretching at 293 K (Fig. la) the whole polymer does not became oriented. The arcs of the reflections of the texturized material are superimposed on the diffraction
110
E.M.
ANT1POV et al.
Fie. 1. Photo-X-ray patterns of different structural forms of PP: (a) a-phase (specimen 3); (b) y-phase (specimen 4); (c) y-phase (specimen 8); (d) mesophase, 373 K.
(a)
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FIG. 2. Equatorial (a) and meridoneal (b) diffraction patterns of PP fibres, annealed at different temperatures. (1) a-phase, (2-6) y-phase, (7) mesophase, (8) isotropic melt. The diffraction patterns are obtained at 293 (2-6), 373 (7), and 525 K (8).
Structure of PP fibres
111
rings of the isotropic polycrystal, the proportion of one or the other being approximately the same, and equal to ~50%. The degree of orientation is thus fairly high; the azimuthal half-width of the reflections gives a value of ~7.5 °. Figure 3a shows the evolution of the changes in the equitorial scattering pictures of the a-phase of PP (specimen 3) on heating under isometric conditions. The passage into the mesomorphic state, according to these data, takes place at 343 K. The X-ray patterns of the mesomorphic state of PP shows the presence of a very intense first maximum and three much weaker reflections on the equator, and strong but wide reflections on the first layer line (Fig. ld). Diffraction patterns of this type are typical of the mesomorphic state, described in particular for polydiethylsiloxane [6] and polyethylene [7]. This structure fits the definition of a conformationally disordered crystalline state given by Wunderlich and Grebowiez [8]. We were able to establish [9-11] that the structure of the PP mesophase shows a uniform layer packing with different levels of order along each of the three measurements. The upper point of existence of the mesomorphic state of PP is 525 K. The transverse dimensions of the regions of coherent scattering exceed 60 nm, but it must be emphasized that the lower limit of the mean value is determined by an X-ray method. On passing into the mesophase the deorientation of the macromolecules increases significantly. The azimuthal half-width of the reflections was increased from 7.5 to ~20 °, which is associated with a partial loss of orientational order at the level of the chain segments on conformational disordering. It should also be noted that a fibre tightly wound
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FiG. 3. Change in scattering pictures of crystalline modifications of PP on heating: (a) a-phase, (100) and (010) reflections (1) 293, (2) 313, (3) 323, (4) 333, (5) 338, (6) 343 K; (b) "?-phase, (200), (010) and (110) reflections (1) 293, (2) 343, (3) 348, (4) 360, (5) 363, and (6) 365 K.
112
E . M . ANTIPOVet al.
onto a frame (isometric conditions) showed appreciable sagging after annealing above the transition point. Annealing above the temperature of transition into the mesomorphic state with subsequent cooling leads to the formation of the y-crystalline modification of PP. Figure lb and Fig. 2a and b (diffraction pattern 2) show the corresponding X-ray patterns for specimen 4. Attention is drawn to the appreciable change in the degree of orientation after heat treatment. The azimuthal half-width of the reflections is 17.5 °, which is close to the corresponding value in the mesomorphic state. It is natural to assume that the strong interaction of the side groups with three fluorine atoms at the ends promotes disordering of the PP chains, and at sufficiently high temperatures can result in a transition from the folded conformation of the macromolecules to more straightened conformations, which, in our view, is observed in the transition into the mesophase. Magill et al. [5] make a similar assumption after analysing the morphological data. Figure 3b illustrates the change in the scattering picture of the PP "r-phase on repeated heating. Up to 343 K the intensities of the first three reflections are somewhat increased, which is characteristic of the annealing process. Over the range 343-360K the intensity of the (110) reflection is decreased, whereas the height of the first two peaks continues to increase. This redistribution of intensities possibly indicates the appearance of texture in the specimen on heating under isometric conditions. Finally, over a narrow temperature range (360-365 K) the intensity of the first reflection begins to increase sharply, and the height of the second is decreased. Over this temperature range the coexistence of crystalline and mesomorphic components is observed, which is in agreement with the data given by Schneider et al. [12]. The intensity of the main maximum of the mesophase is more than an order of magnitude greater than that of the equitorial reflections of the crystalline modifications (Fig. 3). The mean dimension of the ordered regions in passing into the mesophase is increased at least by a factor of three. The plots of S -- f~°I(20) d(20) against temperature for different structural forms of PP (Fig. 4) indicate a sharp "transfer" of the scattering intensity into the region of relatively small diffraction angles (7-12 °) when the crystalline modifications pass into the mesophase. This process is prolonged on further heating of PP in the mesomorphic state up to the isotropization point. Analysis of the results by NMR and DSC [13, 14] enabled the authors to suggest that PP in the mesophase state consists of two phases, with amorphous and mesophase components in metastable equilibrium. However, in our view, another possibility is not excluded: in the mesomorphic state the polymer is a single phase system, but its structure is mesophase [9, 11], so that it simultaneously shows the properties of a crystal and an amorphous substance (poor intra-layer packing). In this case the concept, like such a characteristic as the degree of crystallinity, loses any significance for a mesophase of such a type, in the traditional understanding. In passing into the mesophase spontaneous straightening of the chains occurs. At the transition point the ordering process at the straightened chains as at the nuclei involves almost the whole of the polymeric material in the structural reorganization, with formation of a domain of single phase, mesomorphic structure. Redistribution of the grain boundaries formed by statistical defects as a result of defect migration is the reason for the increase in size of the ordered regions on further heating. Similar phenomena have been observed, in particular the PTFE mesophase [15]. The increase in area of the main mesophase reflection on the scattering curve on heating (Fig. 4) indicates firstly active improvement of the inter-layer ordering (only short-range order is preserved within the limits of the layers), and also an increase in extent of the action of undimensional long-range order in the system, i.e. an increase in size of the domains. In other words, the
Structure of PP fibres
113
defectiveness of the system as a whole on heating from the point of transition into the mesophase state up to the region preceding isotropization is decreased. Analysis of the dimensions of the coherent scattering region [11] as a function of temperature also indicates that the system is single phase above the point of transition into the mesophase. On heating in the first cycle up to a temperature close to the isotropization point the transverse dimension of the ordered regions of the mesophase is increased, and is not changed on cooling, within the limits of measurement error. In contrast to PP, with most flexible chain polymers [15], on cooling in the first cycle this parameter continues to increase, which indicates that the amorphous melt has a maximum annealing temperature and that the ordering of this melt on cooling produces an additional increase in dimensions of the crystallites. The fact that a similar effect is not observed with PP indicates that there is no isotropic phase in this polymer. In our view the existence of a mesophase and an isotropic melt is possible only in the temperature region directly preceding PP isotropization. Crystallization of the mesophase on cooling involves a random nucleus formation mechanism, which results in break up of the large ordering regions into relatively small crystallites with transverse dimensions of the order of 25 nm. Moreover, disorientation of the crystallites formed must correspond to the degree of orientational scatter of the segments of conformationally disordered chains in the mesophase, which is in fact observed. It is also characteristic that the proportion of isotropic polycrystalline component after heat treatment is appreciably decreased (Fig. lb). About 90% of the PP crystallites are now preferentially oriented with the "c" axis along the stretching direction, although, as mentioned earlier, their orientation is lower than in the original specimen. The final state of the specimen after cooling is determined by the maximum annealing temperature in the cycle. Figure 2 shows the changes in equatorial and meridoneal diffraction patterns after annealing at different temperatures. On the equitorial diffraction patterns, with increase in annealing temperature there is a certain redistribution of intensity of the reflections (although in general the number of reflections corresponding to the ~/-lattice is retained), indicating perfection of the PPcrystal structure. The effect of annealing on the meridoneal scattering pictures is especially clear, where, beginning from 433 K, the (001) reflection is resolved. Evaluation of the longitudinal dimensions of the crystallites from the half-width of the given maximum gives the minimum value of the quantity as --70 nm. This is confirmation of the presence of crystals with straightened chains in such a system. Attempts have naturally been made to evaluate the PP large period in different temperature regions. However, it was found that small angle scattering is diffusional in character. There is no discreet maximum in the small angles on the diffraction patterns of the three structural forms of PP, which is in agreement with the results obtained by Russel [16]. In this system the value of the large period evidently lies beyond the resolution limit of the equipment used. Specimens 5-8 differ in the temperatures at which additional thermal stretching is produced with the object of obtaining a maximum increase in the degree of fibre orientation. The photo-X-ray patterns of these specimens are qualitatively identical (Fig. lc shows a typical photograph). The azimuthal half-widths AS of the reflections for all the specimens are shown in Table 1. As can be seen, in spite of the increase in extension ratio up to 12, the value of AS is little changed, and in the limiting case attains a value characteristic of the original specimen stretched at room temperature. Accordingly, the maximum degree of orientation of the PP macromolecules can be obtained by uniaxial stretching both at room temperature and in the mesophase state close to the isotropization
PS 32:1-H
114
E . M . ANTIPOV et al.
p x 10 -3 kg/m 3
~, rel.
degrees max
f,so~
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I 37~
I 473
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FIG. 5.
Equatorial scattering S for PP as a functionof temperature on heating of a-(l) and 1,-(2)crystalline modificationsof PP. Density (1) and position of equitorial (2) and meridoneal (3) diffusion reflections of PP as a function of temperature.
point. However, the proportion of oriented material is essentially different: only half the PP crystallites are preferentially oriented along the stretching axis in the first case and about 100% in the second. Analysis of the X-ray data shows that scattering from the amorphous component on the photo-X-ray patterns of highly oriented specimens after crystallization from the mesomorphic state cannot be detected. The proportion of oriented polymer, the degree of orientation, and the degree of crystallinity have the maximum values for specimen 8. The PP scattering picture can be provisionally separated into two main regions (Fig. 2). The range of diffraction angles 7-10 ° corresponds to interferences on the macromolecule skeletons, and the range 15-25 ° mainly to intra-molecular periodicity, and also to scattering at large groups. As an illustration, Fig. 5 shows the positions of the diffraction maxima of the wide angle diffraction region (15-25 °) on the equator and the meridean of the X-ray pattern for the temperature region corresponding to the existence of a mesophase. In these two cases the difference in the position of the diffusion maximum possibly indicates the end of lateral encompassing of adjacent macromolecules. The density values calculated from crystallographic data for the temperature regions where a mesophase and an isotropic melt exist are given here. The isotropization point is indicated by a break in continuity on each of the three plots of Fig. 5. It can be established that the structure of uniaxially oriented PP fibres after annealing above the point of transition into the mesophase undergoes radical changes. The type of packing is changed, i.e. the non-equilibrium a-phase passes into the thermodynamically stable 7-orhorhombic form. The conformation of the macromolecules is thus changed from the folded conformation characteristic of materials with spherulitic morphology, to a straightened conformation, characteristic of crystals with straightened chains. The limiting degree of orientation of such fibres can be attained on uniaxial stretching both below the point of transition into the mesophase, and in the mesomorphic state in direct proximity to the isotropization temperature, but the degree of defectiveness of the fibres will differ. Translated by N. STANDEN
Structure of PP fibres
115
REFERENCES 1. S. V. VINOGRADOVA, D. R. TUR, I. I. MINOSYANTS, O. L. LIPENDINA, N. I. LARINA and V. V. KORSHAK, Acta Polymerica 33: 598, 1982. 2. D. R. TUR, V. V. KORSHAK, S. V. VINOGRADOVA, G. I. TIMOFEEVA, Ts. A. GOGUADZE, N. O. ALIKHANOVA, A. I. TARASOV and V. I. DUBOVITSKII, Polikondensatsionnye protsessy 85 (Polycondensation Processes-85). Sofia, 1986. 3. M. KOJINA and J. H. MAGILL, Polymer 26: 1971, 1985. 4. M. KOJIMA and J. H. MAGILL, Makromolek. Chem. B186: 649, 1985. 5. J. H. MAGILL, J. PETERMANN and U. RIECK, Colloid and Polymer Sci. 264: 570, 1986. 6. D. Ya. TSVANKIN, V. S. PANKOV, V. P. ZUKOV, Yu. K. GODOVSKII, V. S. SVISTUNOV and A. A. ZHDANOV, J. Polymer Sci. Polymer Chem. Ed. 23: 1043, 1985. 7. V. P. POPOV, E. M. ANTIPOV, S. A. KUPTSOV, N. N. KUZMIN, L. I. BEZRUK and S. Ya. FRENKEL, Acta Polymerica 36: 13, 1985. 8. B. WUNDERLICH and J. GREBOWICZ, Advances Polymer Sci. 60/61: 2, 1984. 9. N. A. PLATE, V. G. KULICHIKHIN, E. M. ANTIPOV and D. R. TUR, Makromolek. Chem. B189: 1447, 1988. 10. E. M. ANTIPOV, S. A. KUPTSOV, Y. G. KULICHIKHIN, D. R. TUR and N. A. PLATE, Makromolek. Chem. Macromolec. Symp. 26: 69, 1989. 11. E. M. ANTIPOV, V. T. KULICHIKItlN, E. K. BORISENKOVA, D. R. TUR and N. A. PLATE, Vysokomol. soyed. A31: No. 11, 1989 (Translated in Polymer Sci. U.S.S.R. 31: 11, 1989). 12. N. S. SCHNEIDER, G. R. DESPER and R. E. SINGLER, J. Appl. Polymer Sci. 20: 3087, 1976. 13. S. M. BISHOP and J. H. HALL, Brit. Polymer J. 6: 193, 1974. 14. V. M. LITVINOV, Y. S. PANKOV and D. R. TUR, Vysokomol. soyed. T28: 289, 1986 (Not translated in Polymer Sci. U.S.S.R.). 15. N. G. SHIRINA, L. Ya. Karpov Physical Chemistry Research Institute, Moscow, 1986. 16. T. P. RUSSEL, D. P. ANDERSON, R. S. STEIN, C. R. DESPER, J. J. BERES and N. S. SCHNEIDER, Macromolecules 17: 1795, 1984.
0032-3950/90 $10.00 + .00 © 1991 Pergamon Press plc
STRUCTURE OF POLY-bisTRIFLUOROETHOXYPHOSPHAZENE FIBRES* E. M. ANTIPOV, V. G. KULICHIKHIN, L. K. GOLOVA, N. P. KRUCHININ, D. R. TUR and N. A. PLATt~ "Khimvolokno" Scientific Production Association; and A. N. Nesmeyanov Institute of Heteroorganic Compounds, Academy of Sciences of the U.S.S.R.
(Received 1 August 1988)
The structural transformations during phase transitions of poly-b/s-trifluoroethoxyphosphazene fibres during heating-cooling cycles are studied by X-ray diffraction. The evolution of the changes in structural parameters is followed as a function of the stretching and heat treatment conditions. It is shown that the defective character of the system is largely determined by the fibre annealing temperature. It is assumed that the polymer macromolecules are spontaneously straightened in the mesomorphic state.
THE USE of polymers as fibres provides a means of solving a great variety of problems in establishing specialized composite materials. Poly-b/s-trifluoroethoxyphosphazene (PP) is a multi-purpose polymer, often used in the form of fibres and fabrics. Since the properties of fibrous materials are largely determined by the special features of their structural organization, the necessity of studying the structure of PP as a function of the degree of orientation attained in cold and hot stretching is obvious. In view of this, the main problem in this work is a study of the special structural features of fibres obtained from a solution of high molecular PP as a function of the stretching and heat treatment conditions. The method described by Vinogradova et al. [1] was used to synthesize the PP. The molecular weight, as evaluated from the intrinsic viscosity of a solution in THF[2] was 1.2x 107; the polydispersity factor Mw/Mn, as determined from the MMD distribution curve did not exceed 1.4. The PP fibres were obtained by a wet method from a 3% solution in ethyl acetate. A mixture of CC14 and ethyl acetate was used as precipitant. The dried PP fibres were subjected to annealing and orientational stretching at various temperatures. The conditions of obtaining the specimens are shown in Table 1. In the X-ray diffraction studies, the large angle photo-X-ray diffraction patterns were obtained with a IRIS-3.0 apparatus (CuK~, Ni filter, fiat cell). The diffraction patterns were obtained with a DRON-3.0 instrument (CuK~, curved, focusing quartz crystal-monochromator in the primary beam) using transillumination photography. The diffraction patterns were recorded along the equator and along the meridian of the photo-X-ray patterns. The photographs of the oriented specimens at different temperatures were obtained under isometric conditions; the fibres were wound onto a special frame-holder and fixed with clamps at the edges. The small angle diffraction patterns were obtained with a KRM-1 apparatus with an automated system for controlling and processing the data. Photography was carried out with slit collimation along the meridian of the X-ray patterns, at a resolution of five angular minutes. * Vysokomol. soyed. A32: No. 1, 108-115, 1990.
108
Structure of PP fibres
109
Depending on the molecular mass and the conditions of obtaining the material the PP can crystallize with the formation of three types of crystal lattice, i.e. a,/3, and y [3, 4]. It is shown that the a- and y-phases are orthorhombic in type with two and four monomer units in the elementary cell, respectively, whereas the /3-modification is monoclinic, which is characteristic only of the crystals of low molecular PP. In the region of 343-520 K the PP is in the mesophase state.
TABLE 1.
Specimen number 1 2 3 4 5 6 7 8
CHARACTERISTICS OF PP SPECIMENS
Specimen characteristics
Isotropicfilm from solution Fibre without orientationalstretching Specimen2, subjected to orientationai stretchingat 293 K Specimen3 after annealingat 433 K on a rigid frame over 1 hour Specimen3, subjected to hot stretching at 448 K Specimen3, subjected to hot stretching at 453 K Specimen6, subjectedto hot stretchingat 473--483 K Specimen7, subjected to hot stretching at 523 K
Extension ratio, ;t
Azimuthal half-width A~, deg
0 0 3.4 3.4 6.5 6.8 8.0 10-12
-30 7.5 17.5 9.0 8,5 8.0 7.5
The wide angle diffraction patterns of specimen 1 (Table 1) indicate the partially crystalline nature of the polymer film obtained by casting from solution. Analysis of the diffraction data showed that in this case PP is crystallized in a non-equilibrium a-lattice. The process evidently involves by-passing the preliminary stage in which a mesophase appears, in contrast to crystallization of the polymer from the melt with a formation of a y-phase. In the first case relatively small crystallites are obtained with a-orthorhombic packing of the chains, which, according to previous authors [3, 4] are in the folded conformation. The temperature of transition of the a-phase into the mesomorphic state is - 2 0 K lower than the corresponding temperature of the equilibrium y-modification. There is no preferential orientation of the macromolecules in the PP films. Evaluation of the dimensions of the crystalline formations from the half-widths of the diffraction lines gives the value 15-20 nm. According to data provided by Magill et al. [15] this polymer has spherulitic supermolecular organization. Determination of the degree of crystallinity is difficult because of the impossibility of separating the scattering from the amorphous and crystalline components. The half-width of the profile of the amorphous halo is probably relatively small, and is screened by the crystal reflections. The density of crystalline packing of the a-phase, as determined from crystallographic data at 293 K, is 1780 kg/m 3, i.e. somewhat higher than for the y-modification (1732 kg/m3), which is natural for a system with half the elementary cell. However, because of the low degree of crystalinity of the non-equilibrium a-lattice the overall density of specimens in the various cases is approximately the same, and according to Refs [3, 4] is 1715 kg/m 3. A PP fibre formed from solution without plastificational stretching (specimen 2) shows weak signs of anisotropy, which indicates the initial stage of molecular orientation. Uniaxial stretching of such a specimen at 293 K results in the appearance of an axial texture (specimen 3) (Fig. la, and Fig. 2, diffraction pattern 1). It should be noted that after stretching at 293 K (Fig. la) the whole polymer does not became oriented. The arcs of the reflections of the texturized material are superimposed on the diffraction
110
E.M.
ANT1POV et al.
Fie. 1. Photo-X-ray patterns of different structural forms of PP: (a) a-phase (specimen 3); (b) y-phase (specimen 4); (c) y-phase (specimen 8); (d) mesophase, 373 K.
(a)
(b)
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FIG. 2. Equatorial (a) and meridoneal (b) diffraction patterns of PP fibres, annealed at different temperatures. (1) a-phase, (2-6) y-phase, (7) mesophase, (8) isotropic melt. The diffraction patterns are obtained at 293 (2-6), 373 (7), and 525 K (8).
Structure of PP fibres
111
rings of the isotropic polycrystal, the proportion of one or the other being approximately the same, and equal to ~50%. The degree of orientation is thus fairly high; the azimuthal half-width of the reflections gives a value of ~7.5 °. Figure 3a shows the evolution of the changes in the equitorial scattering pictures of the a-phase of PP (specimen 3) on heating under isometric conditions. The passage into the mesomorphic state, according to these data, takes place at 343 K. The X-ray patterns of the mesomorphic state of PP shows the presence of a very intense first maximum and three much weaker reflections on the equator, and strong but wide reflections on the first layer line (Fig. ld). Diffraction patterns of this type are typical of the mesomorphic state, described in particular for polydiethylsiloxane [6] and polyethylene [7]. This structure fits the definition of a conformationally disordered crystalline state given by Wunderlich and Grebowiez [8]. We were able to establish [9-11] that the structure of the PP mesophase shows a uniform layer packing with different levels of order along each of the three measurements. The upper point of existence of the mesomorphic state of PP is 525 K. The transverse dimensions of the regions of coherent scattering exceed 60 nm, but it must be emphasized that the lower limit of the mean value is determined by an X-ray method. On passing into the mesophase the deorientation of the macromolecules increases significantly. The azimuthal half-width of the reflections was increased from 7.5 to ~20 °, which is associated with a partial loss of orientational order at the level of the chain segments on conformational disordering. It should also be noted that a fibre tightly wound
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FiG. 3. Change in scattering pictures of crystalline modifications of PP on heating: (a) a-phase, (100) and (010) reflections (1) 293, (2) 313, (3) 323, (4) 333, (5) 338, (6) 343 K; (b) "?-phase, (200), (010) and (110) reflections (1) 293, (2) 343, (3) 348, (4) 360, (5) 363, and (6) 365 K.
112
E . M . ANTIPOVet al.
onto a frame (isometric conditions) showed appreciable sagging after annealing above the transition point. Annealing above the temperature of transition into the mesomorphic state with subsequent cooling leads to the formation of the y-crystalline modification of PP. Figure lb and Fig. 2a and b (diffraction pattern 2) show the corresponding X-ray patterns for specimen 4. Attention is drawn to the appreciable change in the degree of orientation after heat treatment. The azimuthal half-width of the reflections is 17.5 °, which is close to the corresponding value in the mesomorphic state. It is natural to assume that the strong interaction of the side groups with three fluorine atoms at the ends promotes disordering of the PP chains, and at sufficiently high temperatures can result in a transition from the folded conformation of the macromolecules to more straightened conformations, which, in our view, is observed in the transition into the mesophase. Magill et al. [5] make a similar assumption after analysing the morphological data. Figure 3b illustrates the change in the scattering picture of the PP "r-phase on repeated heating. Up to 343 K the intensities of the first three reflections are somewhat increased, which is characteristic of the annealing process. Over the range 343-360K the intensity of the (110) reflection is decreased, whereas the height of the first two peaks continues to increase. This redistribution of intensities possibly indicates the appearance of texture in the specimen on heating under isometric conditions. Finally, over a narrow temperature range (360-365 K) the intensity of the first reflection begins to increase sharply, and the height of the second is decreased. Over this temperature range the coexistence of crystalline and mesomorphic components is observed, which is in agreement with the data given by Schneider et al. [12]. The intensity of the main maximum of the mesophase is more than an order of magnitude greater than that of the equitorial reflections of the crystalline modifications (Fig. 3). The mean dimension of the ordered regions in passing into the mesophase is increased at least by a factor of three. The plots of S -- f~°I(20) d(20) against temperature for different structural forms of PP (Fig. 4) indicate a sharp "transfer" of the scattering intensity into the region of relatively small diffraction angles (7-12 °) when the crystalline modifications pass into the mesophase. This process is prolonged on further heating of PP in the mesomorphic state up to the isotropization point. Analysis of the results by NMR and DSC [13, 14] enabled the authors to suggest that PP in the mesophase state consists of two phases, with amorphous and mesophase components in metastable equilibrium. However, in our view, another possibility is not excluded: in the mesomorphic state the polymer is a single phase system, but its structure is mesophase [9, 11], so that it simultaneously shows the properties of a crystal and an amorphous substance (poor intra-layer packing). In this case the concept, like such a characteristic as the degree of crystallinity, loses any significance for a mesophase of such a type, in the traditional understanding. In passing into the mesophase spontaneous straightening of the chains occurs. At the transition point the ordering process at the straightened chains as at the nuclei involves almost the whole of the polymeric material in the structural reorganization, with formation of a domain of single phase, mesomorphic structure. Redistribution of the grain boundaries formed by statistical defects as a result of defect migration is the reason for the increase in size of the ordered regions on further heating. Similar phenomena have been observed, in particular the PTFE mesophase [15]. The increase in area of the main mesophase reflection on the scattering curve on heating (Fig. 4) indicates firstly active improvement of the inter-layer ordering (only short-range order is preserved within the limits of the layers), and also an increase in extent of the action of undimensional long-range order in the system, i.e. an increase in size of the domains. In other words, the
Structure of PP fibres
113
defectiveness of the system as a whole on heating from the point of transition into the mesophase state up to the region preceding isotropization is decreased. Analysis of the dimensions of the coherent scattering region [11] as a function of temperature also indicates that the system is single phase above the point of transition into the mesophase. On heating in the first cycle up to a temperature close to the isotropization point the transverse dimension of the ordered regions of the mesophase is increased, and is not changed on cooling, within the limits of measurement error. In contrast to PP, with most flexible chain polymers [15], on cooling in the first cycle this parameter continues to increase, which indicates that the amorphous melt has a maximum annealing temperature and that the ordering of this melt on cooling produces an additional increase in dimensions of the crystallites. The fact that a similar effect is not observed with PP indicates that there is no isotropic phase in this polymer. In our view the existence of a mesophase and an isotropic melt is possible only in the temperature region directly preceding PP isotropization. Crystallization of the mesophase on cooling involves a random nucleus formation mechanism, which results in break up of the large ordering regions into relatively small crystallites with transverse dimensions of the order of 25 nm. Moreover, disorientation of the crystallites formed must correspond to the degree of orientational scatter of the segments of conformationally disordered chains in the mesophase, which is in fact observed. It is also characteristic that the proportion of isotropic polycrystalline component after heat treatment is appreciably decreased (Fig. lb). About 90% of the PP crystallites are now preferentially oriented with the "c" axis along the stretching direction, although, as mentioned earlier, their orientation is lower than in the original specimen. The final state of the specimen after cooling is determined by the maximum annealing temperature in the cycle. Figure 2 shows the changes in equatorial and meridoneal diffraction patterns after annealing at different temperatures. On the equitorial diffraction patterns, with increase in annealing temperature there is a certain redistribution of intensity of the reflections (although in general the number of reflections corresponding to the ~/-lattice is retained), indicating perfection of the PPcrystal structure. The effect of annealing on the meridoneal scattering pictures is especially clear, where, beginning from 433 K, the (001) reflection is resolved. Evaluation of the longitudinal dimensions of the crystallites from the half-width of the given maximum gives the minimum value of the quantity as --70 nm. This is confirmation of the presence of crystals with straightened chains in such a system. Attempts have naturally been made to evaluate the PP large period in different temperature regions. However, it was found that small angle scattering is diffusional in character. There is no discreet maximum in the small angles on the diffraction patterns of the three structural forms of PP, which is in agreement with the results obtained by Russel [16]. In this system the value of the large period evidently lies beyond the resolution limit of the equipment used. Specimens 5-8 differ in the temperatures at which additional thermal stretching is produced with the object of obtaining a maximum increase in the degree of fibre orientation. The photo-X-ray patterns of these specimens are qualitatively identical (Fig. lc shows a typical photograph). The azimuthal half-widths AS of the reflections for all the specimens are shown in Table 1. As can be seen, in spite of the increase in extension ratio up to 12, the value of AS is little changed, and in the limiting case attains a value characteristic of the original specimen stretched at room temperature. Accordingly, the maximum degree of orientation of the PP macromolecules can be obtained by uniaxial stretching both at room temperature and in the mesophase state close to the isotropization
PS 32:1-H
114
E . M . ANTIPOV et al.
p x 10 -3 kg/m 3
~, rel.
degrees max
f,so~
"
=
~,4o~~ ..
I 37~
I 473
FIG. 4. FIG. 4. FIG. 5.
J f
T,K
3~'3
-
-
2~
I 473
FIG. 5.
Equatorial scattering S for PP as a functionof temperature on heating of a-(l) and 1,-(2)crystalline modificationsof PP. Density (1) and position of equitorial (2) and meridoneal (3) diffusion reflections of PP as a function of temperature.
point. However, the proportion of oriented material is essentially different: only half the PP crystallites are preferentially oriented along the stretching axis in the first case and about 100% in the second. Analysis of the X-ray data shows that scattering from the amorphous component on the photo-X-ray patterns of highly oriented specimens after crystallization from the mesomorphic state cannot be detected. The proportion of oriented polymer, the degree of orientation, and the degree of crystallinity have the maximum values for specimen 8. The PP scattering picture can be provisionally separated into two main regions (Fig. 2). The range of diffraction angles 7-10 ° corresponds to interferences on the macromolecule skeletons, and the range 15-25 ° mainly to intra-molecular periodicity, and also to scattering at large groups. As an illustration, Fig. 5 shows the positions of the diffraction maxima of the wide angle diffraction region (15-25 °) on the equator and the meridean of the X-ray pattern for the temperature region corresponding to the existence of a mesophase. In these two cases the difference in the position of the diffusion maximum possibly indicates the end of lateral encompassing of adjacent macromolecules. The density values calculated from crystallographic data for the temperature regions where a mesophase and an isotropic melt exist are given here. The isotropization point is indicated by a break in continuity on each of the three plots of Fig. 5. It can be established that the structure of uniaxially oriented PP fibres after annealing above the point of transition into the mesophase undergoes radical changes. The type of packing is changed, i.e. the non-equilibrium a-phase passes into the thermodynamically stable 7-orhorhombic form. The conformation of the macromolecules is thus changed from the folded conformation characteristic of materials with spherulitic morphology, to a straightened conformation, characteristic of crystals with straightened chains. The limiting degree of orientation of such fibres can be attained on uniaxial stretching both below the point of transition into the mesophase, and in the mesomorphic state in direct proximity to the isotropization temperature, but the degree of defectiveness of the fibres will differ. Translated by N. STANDEN
Structure of PP fibres
115
REFERENCES 1. S. V. VINOGRADOVA, D. R. TUR, I. I. MINOSYANTS, O. L. LIPENDINA, N. I. LARINA and V. V. KORSHAK, Acta Polymerica 33: 598, 1982. 2. D. R. TUR, V. V. KORSHAK, S. V. VINOGRADOVA, G. I. TIMOFEEVA, Ts. A. GOGUADZE, N. O. ALIKHANOVA, A. I. TARASOV and V. I. DUBOVITSKII, Polikondensatsionnye protsessy 85 (Polycondensation Processes-85). Sofia, 1986. 3. M. KOJINA and J. H. MAGILL, Polymer 26: 1971, 1985. 4. M. KOJIMA and J. H. MAGILL, Makromolek. Chem. B186: 649, 1985. 5. J. H. MAGILL, J. PETERMANN and U. RIECK, Colloid and Polymer Sci. 264: 570, 1986. 6. D. Ya. TSVANKIN, V. S. PANKOV, V. P. ZUKOV, Yu. K. GODOVSKII, V. S. SVISTUNOV and A. A. ZHDANOV, J. Polymer Sci. Polymer Chem. Ed. 23: 1043, 1985. 7. V. P. POPOV, E. M. ANTIPOV, S. A. KUPTSOV, N. N. KUZMIN, L. I. BEZRUK and S. Ya. FRENKEL, Acta Polymerica 36: 13, 1985. 8. B. WUNDERLICH and J. GREBOWICZ, Advances Polymer Sci. 60/61: 2, 1984. 9. N. A. PLATE, V. G. KULICHIKHIN, E. M. ANTIPOV and D. R. TUR, Makromolek. Chem. B189: 1447, 1988. 10. E. M. ANTIPOV, S. A. KUPTSOV, Y. G. KULICHIKHIN, D. R. TUR and N. A. PLATE, Makromolek. Chem. Macromolec. Symp. 26: 69, 1989. 11. E. M. ANTIPOV, V. T. KULICHIKItlN, E. K. BORISENKOVA, D. R. TUR and N. A. PLATE, Vysokomol. soyed. A31: No. 11, 1989 (Translated in Polymer Sci. U.S.S.R. 31: 11, 1989). 12. N. S. SCHNEIDER, G. R. DESPER and R. E. SINGLER, J. Appl. Polymer Sci. 20: 3087, 1976. 13. S. M. BISHOP and J. H. HALL, Brit. Polymer J. 6: 193, 1974. 14. V. M. LITVINOV, Y. S. PANKOV and D. R. TUR, Vysokomol. soyed. T28: 289, 1986 (Not translated in Polymer Sci. U.S.S.R.). 15. N. G. SHIRINA, L. Ya. Karpov Physical Chemistry Research Institute, Moscow, 1986. 16. T. P. RUSSEL, D. P. ANDERSON, R. S. STEIN, C. R. DESPER, J. J. BERES and N. S. SCHNEIDER, Macromolecules 17: 1795, 1984.